Looking at the website, the PRS 650 is available for 199.99. If there is not waiting list (as that model is notoriously popular), I would recommend that as well, mainly because it is 6 in. There's a reason that 6 in readers are the most common ones. It fits more information in and it's more useful in case you have to read PDF in landscape mode.
Also, if you are comfortable (and I infer you are some you're a university student and therefore technically inclined), I would install PRS+, a custom firmware for PRS X50, which lets you install custom fonts and add games and other options. Sony Reader + PRS+ is imo the best reading device.
An awesome installation guide is available on
http://www.anamardoll.com/2011/06/er...tion-with.html